The invention relates to sensors employing surface acoustic wave ("SAW") devices. An important aspect of the invention relates to using SAW devices to measure pressure, such as hydrostatic pressure.
Sensors employing SAW devices (such as delay lines and resonators), have been proposed for use in measuring parameters such as acceleration and pressure. SAW sensors rely on the propagation of acoustic waves in media which respond to external influences the effect of which changes SAW characteristics such as wave velocity or frequency. In one prior proposal, a SAW device is on a thin diaphragm mounted so as to flex under the external influences to be measured and thereby change the characteristics of the SAW device in a way which can be measured. Another type of a previously proposed sensor structure uses a cylindrical body as an externally loaded or an internally loaded probe. For an externally loaded structure, the body has a sealed cavity, with one or more SAW devices mounted on the interior surface. When external pressure is applied, the body flexes and changes the SAW device characteristics. For an internally loaded structure, one or more SAW devices are on the exterior surface of the cylindrical body, and outward pressure is applied at a central opening in the cylinder. A detailed description of surface acoustic wave sensors can be found in commonly assigned co-pending application Ser. No. 427,240 filed on Sept. 29, 1982 now U.S. Pat. No. No. 4,535,631. The entire contents of said 1982 application are hereby incorporated by reference herein, and the prior art discussed or cited in said 1982 application is hereby brought to the attention of the Examiner herein. Improvements of such devices are disclosed in commonly owned application Ser. No. 687,715 filed concurrently herewith, the entire contents of which are hereby incorporated by reference herein, and the prior art discussed or cited in which is hereby brought to the attention of the Examiner.
One way of making externally loaded structures is to start with a solid cylinder of a material such as crystalline quartz, saw it in half along an axial plane and rout the flat sides of the halves to form channels with respective "flats" at their bottoms. The term "flat" is used here to mean a surface which is flat enough to form a SAW device thereon; in fact, the surface can be planar or curved. A respective SAW device is formed on each flat, and the two halves are reassembled into a cylinder and secured to each other, as by suitable bonding, to seal the internal cavity formed by the facing channels. Electrical leads from the SAW resonators run through the bond to an external circuit. This structure typically is mounted in a housing which allows the structure to be selectively subjected to external pressure, such as pressure from fluid which is selectively admitted in the housing. The change in the difference frequency of the two SAW devices, as between the steady states before and after the pressure was applied, is used as a measure of the pressure change. Examples of such structures are described in said commonly assigned, earlier-filed application.
An exemplary and nonlimiting use of structures of this type is in sensing downhole pressure in exploratory or producing oil wells. Stringent and often conflicting requirements are imposed on such structure by the severe downhole conditions, such as high pressure and temperature and the need to measure accurately very small pressure differentials. As one example, in evaluating and planning the exploitation of producing wells, it is sometimes desirable to change the producing rate of one well in a field (e.g., close off that wall temporarily), and to measure and time the resulting pressure changes in one or more other wells in the same field, for example in order to estimate the permeability of the subsurface formations. It will be appreciated that this requires relatively minute changes in pressure to be accurately measured under the difficult downhole conditions.
Accordingly, it would be desirable for SAW pressure sensors to provide pressure measurement capabilities with a dynamic range of about a million or more, pressure response time of the order of several seconds or less over a temperature range of 0.degree.-125.degree. C. or more, thermal response time which is as short as possible, and high accuracy. Stated differently, it is desirable to have a SAW pressure sensor having a pressure range of 0-10,000 psi, resolution of 0.01 psi, thermal response time of about 10 seconds or less per 100.degree. C. step change in ambient temperature, and accuracy of 0.025% full scale. It is further desirable to exceed even these goals, and provide a structure having a pressure range exceeding 0-20,000 psi, temperature range exceeding 0.degree.-175.degree. C. and pressure and temperature response time on the order of a few seconds, without compromising the abovementioned resolution and accuracy.
The above-identified 1982 application discloses among other things an externally loaded SAW pressure sensor which has a cylindrical shape and an internal cavity in which two SAW resonators are formed in channels facing each other. An external circuit measures the difference frequency of the SAW resonators, and uses it as a temperature-compensated measure of the pressure on the external surface of the structure. Examples of such structures are illustrated in FIGS. 4, 5, 12, 13, 14, 15, 19, 20, 29, 30 and 33 of the commonly assigned earlier-filed application. In each of these illustrations of particular embodimemts, the SAW resonator is on a flat surface (called simply a "flat") in the bottom of a channel, and the channel sides which flank the long sides of the resonator flat are perpenicular to the flat, and are substantially straight and planar, except possibly for some rounding of sharp corners which are designed to relieve stresses at wall intersections. While the inventions claimed therein need not be limited to straight channel sidewalls, no figure of said earlier-filed application illustrates a curved sidewall immediately flanking the SAW device flat in an externally loaded structure or such walls which are at an angle relative to the flat which is substantially different from the normal.
It is believed that the structures disclosed and claimed in the 1982 application provide significant improvement over prior proposals. Still, a need remains for even greater improvements. In an effort to meet that need, it has been discovered that unexpected and highly significant improvements result from: (i) changing the structure configuration by curving or rounding the sidewall immediately flanking one or more of the respective SAW devices, or having angling that sidewall at much more than 90.degree. relative to the flat; and/or (ii) flattening the outside surface of the sensor adjacent one of the interior flats so that the wall thicknesses at the two flats make their thermal responses better matched to each other. These improvements include an unexpected and significant increase in pressure sensitivity without significant adverse effects on stresses, and an unexpected and significant improvement in thermal response without unacceptably degrading pressure sensitivity.
Accordingly, an exemplary and nonlimiting embodiment of the invention comprises a pressure sensor in the form of a generally cylindrical body made of two halves joined along an axial plane. An axially extending channel in each half faces that of the other, and has an axially extending flat at its bottom. A respective SAW device occupies each flat. At least one of the channels can have a curved sidewall adjoining the flat or sidewalls which angle away from the long sides of the flat at an angle substantially exceeding 90.degree.. Preferably, but not necessarily, the curved sidewall conforms to a cylindrical surface. The curved sidewall can be formed with a routing tool having a curved or a cylindrical surface. The two channels can differ in depth, such that the wall thickness of the body can be greater at one flat than at the other (at least when the sensor is being fabricated), and the flats can differ in width. The outer surface of the sensor adjacent one of the flats can be flattened, to make the thermal responses at the flats better matched without unacceptable degrading of pressure sensitivity.
In broader terms, an exemplary and nonlimiting example of the invention comprises a hollow body having at least one SAW device which is formed on its interior surface and whose frequency varies with pressure exerted on the exterior of the body. At least a substantial portion of the interior surface adjoining the SAW device can be curved, or can angle away from the SAW device at an angle substantially exceeding 90.degree.. Preferably, the hollow body has two interior flats which are formed on the interior thereof and face each other. The SAW devices can be formed on respective interior flats. The wall thicknesses of the sensor body at the flats can be matched for better matching of thermal response as between the respective SAW devices.
A pressure sensitivity increase of the order of 50% is estimated for a particular embodiment having a curved sidewall adjacent one flat, as compared to a particular embodiment having planar sidewalls adjacent both flats, without significant increase in stresses at the flat centers. A shortening of the thermal response time by at least about one-third is estimated where the wall thicknesses of the sensor body at the flats are matched, without unacceptable degrading of pressure sensitivity or an unacceptable increase in stresses.
These and other advantages of the invention are discussed in more detail below in connection with the exemplary embodiments shown in the figures.